Hot melt adhesives are solid at room temperature but, upon application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. Curable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. An advantage of hot melt curable adhesives over traditional liquid curing adhesives is their ability to provide “green strength” upon cooling prior to cure.
The majority of reactive hot melts are moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer. Polyurethane prepolymers are conventionally obtained by reacting diols with diisocyanates. Pure diols are favored for use, instead of polyols with higher functionality, to avoid excessive branching that can lead to poor pot stability. Methylene bisphenyl diisocyanate (MDI) is favored over lower molecular weight isocyanates to minimize volatility. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the adhesive, and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a crosslinked material held together primarily through urea groups and urethane groups.
The prior art discloses that the performance of reactive hot melt adhesives for most applications may be substantially improved by the incorporation of low molecular weight acrylic polymers and/or incorporating crystalline diols, e.g. polyesters. Prior art adhesives are tough, with good low temperature flexibility, heat and chemical resistance, and specific adhesion to polar substrates. Adhesion to a wide range of other substrates may be obtained through the addition of adhesion promoters such as silane coupling agents. However, it is difficult to achieve long open time and/or high green strength at a reasonable application viscosity. High green strength can be achieved by using crystalline materials (e.g polyester diols), however this substantially limits the open time achievable. Alternatively, this can be achieved by the use of polyurethanes with high molecular weight, however the resulting application viscosity is high and the open time is limited. EP246473B discloses the use of low molecular weight acrylic polymers to increase cohesive strength. These low molecular weight polymers are typically in the range of 10,000-30,000 g/mole and contain no active hydrogen. High green strength can also be achieved by using hydroxyl functionalized acrylic polymers, as exemplified by U.S. Pat. No. 5,866,656 (Mn 4,000-15,000) and EP1036103 (Mw<60,000 g/mole) but the molecular weight or concentration should not be too high or the viscosity will be too high and/or the product will have low viscosity stability. This is likely to result from a fraction of the acrylic polymer containing greater than 2 hydroxyl functional groups per chain. This arises from the hydroxyl functionality being provided by a functional comonomer that is incorporated in a statistical fashion during a free-radical polymerisation, providing polymer chains with a distribution of functionality. Despite advances in the art, there remains a need for improvements in reactive hot melt technology to expand the application of such adhesives and their effectiveness in such applications. The present invention addresses this need.